Just had duals put on my 66 Galaxie 352 4brl. No crossover pipe was put in. How important is a crossover pipe? Does it improve performance?

WHat is the best method to determine where the pipe should go? I have heard of guys painting a white stripe from the headers back on the underside of the pipe. Drive it until it burns off then look to see where the paint stopped burning. This is the spot to put the H pipe. Anyone ever tried this or is it an old wives tale?